The present invention relates to the use of semiconductor materials for high frequency high-power applications and in particular relates to the use of Group III nitride materials for high electron mobility transistors (HEMTs), and most particularly to improvements in these material systems that lead to corresponding improvements in the electronic performance of the transistors.
A HEMT is a solid-state transistor that offers advantages for signal amplification in applications such as (but not limited to) radar, cellular telephone communications, satellite communications and other microwave circuit applications.
A HEMT behaves much like a conventional Field Effect Transistor (FET). A conducting channel between drain and source electrodes can be affected by applying a voltage to the gate electrode. This modulates the drain-source current. In a HEMT the conducting channel is created by a heterostructure (two different semiconductor materials adjacent one another). The difference between the hetero materials (particularly their respective bandgaps and electron affinities) must be sufficient to encourage the formation of a thin layer of charge carriers at the hetero interface. The thin layer is typically referred to as a two dimensional electron gas (“2DEG”). The concentration of the carriers and their speed in this layer enables the HEMT to maintain a high gain at very high frequencies.
As with other semiconductor devices, many of the performance parameters of any given HEMT are directly related to the material from which it is formed. Gallium arsenide (GaAs), which has become a more standard material for HEMTs, offers a higher electronic mobility (6000 cm2/V-s) and a lower source resistance than silicon and thus provides devices that can function at higher frequencies than comparable silicon-based devices. Nevertheless, gallium arsenide has a relatively small bandgap (1.42 eV) and a relatively small breakdown voltage which limits the higher power, higher frequency applications with which makes gallium arsenide less suitable, or in some cases unsuitable, for higher power, higher frequency applications.
Accordingly, interest in HEMTs has moved to higher bandgap materials such as the Group III nitride material system. Depending upon the particular composition, Group III nitrides can have bandgaps as high as 6 eV (for AlN), and relatively high electron mobility (up to about 2000 cm2/V-s). Perhaps more importantly with respect to the operation of the HEMT, the aluminum gallium nitride based structure can demonstrate a 2DEG sheet density in excess of 1013 per square centimeter (cm−2).
As noted above, the performance of semiconductor devices relates to the materials from which they are formed and the design and quality of particular layers or substrates of those materials. The lack of quality or uniformity in the materials, or in material structures that form devices or device precursors, reduces the yield and limits the sizes of resulting devices that are potentially available from the material systems.
With respect to size, devices built from non-uniform material also show less linearity and greater variation in threshold voltages. Additionally, epitaxial growth normally produces variations in materials (composition and characteristics) across a wafer. These can include differences in concentration of one or more elements (e.g., aluminum in aluminum gallium nitride) and different thicknesses.
To date, higher quality Group III nitride structures are available on two inch and three inch wafers, but these are less advantageous for larger power devices. Additionally, because the edge of every wafer, regardless of size, typically requires about an 8 mm loss, edge losses are proportionally high for smaller wafers. Furthermore, because 100 mm is a common wafer size for other materials (e.g., gallium arsenide), 100 mm wafers with Group III nitride epilayers can be handled by much existing equipment, thus avoiding re-tooling.
Processing costs, however, are typically similar regardless of the size of a wafer so that smaller wafers increase manufacturing costs per device of a given size.
Although larger wafers are thus theoretically desirable, larger wafers are hard to manage because of the increased tendency to bow or warp and because of the typical epitaxial growth characteristics mentioned above.
Therefore, larger size, higher quality, high consistency performance wafers that include Group III nitride heterostructures remain a worthwhile and desired goal in the semiconductor art.